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Making light work of LED droop

The drive to bring eco-friendly LED lighting into our homes is being stopped in its tracks by an embarrassing problem known as droop – the disappointing reduction in efficiency that happens when the light bulbs operate at the high power levels they need to shine at their brightest.

“Efficiency droop is one of the main obstacles to achieving cost-effective and high-efficiency LEDs,” says Seong-Ju Park at the Gwangju Institute of Science and Technology (GIST) in South Korea. “Droop becomes a very important issue as LEDs expand into applications like [indoor] lighting where they operate at high currents.”

For years, LED production has grown in tandem with the cellphone, providing the backlight for their displays. But manufacturers will have to tackle droop before high-power LEDs can hit the big time.

The cause of LED droop is disputed, making the solution to the problem far from clear – but now, Park and colleagues at GIST have teamed up with Samsung LED to prop up this flagging performance with an unconventional device design.

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A standard LED has a surplus of electrons on one side and a dearth of electrons – or an abundance of electron “holes” – on the other. Plug the LED into a circuit, and the electrons and holes move towards each other, combine, and release energy as light.

Droop means that the proportion of the recombinations that produce light peaks at low electrical powers, with the record-holding prototype devices reaching about 250 lumens per watt. Raise the power to levels typically used for indoor lighting, though, and an increasing proportion of the electric current is lost as heat, so the efficiency drops below 100 lumens per watt.

A trap to catch an electron

The electrons and holes are caught in tiny traps called quantum wells, where they are more likely to collide and recombine. In commercial white LEDs, quantum wells are made of gallium nitride (GaN) surrounded by barriers on either side made from indium gallium nitride (InGaN).

But conventional manufacturing techniques simply juxtapose the InGaN and GaN layers, creating an abrupt interface that physically strains the semiconductor material and generates an electrical field, which Park’s team suggest might cut the chances of electrons and holes combining and emitting light.

To test the idea, the researchers changed the nature of the interface between the well and its barrier by introducing the indium more gradually. That created a steady gradient between the barriers and the well. By smoothing the interface between these layers, they could reduce the strain and thus weaken the electric field surrounding the quantum wells.

When compared against a conventional design, the team’s LED rapidly becomes 20 per cent more efficient as the power goes up, generating more light and less heat. While similarly raised efficiencies have been reported before, the Korean team’s approach reaches these levels at much lower currents and then sustains them.

“The paper appears to be an interesting contribution to the wide-ranging debate on droop,” comments Rachel Oliver from the Centre for Gallium Nitride at the University of Cambridge, UK, However, she warns that LED droop is a complicated issue and unlikely to have a single, simple solution. “To really solve this important problem will require more wide-ranging and systematic studies.”